Conventionally, Brillouin scattered light has been coherently detected by supplying an optical fiber with pulsed light generated by pulsing continuous waveform light to acquire scattered light from the optical fiber (refer to FIG. 8 of Japanese Laid-Open Patent Publication No. 2001-165808). The coherent detection is carried out by multiplexing the scattered light and intensity—modulated light obtained by modulating the continuous waveform light (optical frequency: fc) by an intensity modulation at a predetermined frequency p, for example. It should be noted that scattered light include Rayleigh scattered light (optical frequency: fc) in addition to the Brillouin scattered light (optical frequencies: fc+fb and fc−fb). Moreover, it should be noted that the intensity—modulated light includes a carrier light component having the optical frequency fc, and sideband light components having the optical frequencies fc+p and fc−p. Moreover, the coherent detection is carried out by an optical receiver including a photo diode.
However, according to the above prior art technology, a result of the coherent detection includes a result of the detection of the Rayleigh scattered light (center frequency: 0) in addition to a result of the detection of the Brillouin scattered light (center frequency: |p−fb|). In general, the power of the detection result of the Rayleigh scattered light is larger than the power of the detection result of the Brillouin scattered light. Therefore, if |p−fb| is not sufficiently higher than 0 (zero), the detection result of the Brillouin scattered light will be buried in the detection result of the Rayleigh scattered light, and the Brillouin scattered light thus cannot be measured.
However, if |p−fb| is sufficiently higher than 0, it is necessary to widen the reception band of the photo diode of the optical receiver. A photo diode having a wide reception band is expensive, and the cost for detecting the Brillouin scattered light thus increases.
In view of the foregoing problems, it is an object of the present invention to measure Brillouin scattering light without using an optical receiver having a wide reception band.